Abstract

Plakoglobin (\#947;-catenin) and \#946;-catenin are highly homologous proteins that function in cell-cell adhesion and signaling. Both proteins exist in a cadherin-associated form, which mediates adhesion, and in a distinct soluble, cytosolic/nuclear form, which plays a signaling role. Despite their interactions with common cellular partners, \#946;-catenin has well documented oncogenic potenital while plakoglobin expression is generally associated with tumor suppression. The underlying mechanism(s) for suppression of tumorigenicity by plakoglobin is yet to be defined. We previously showed that plakoglobin expression in the SCC9 oral squamous carcinoma cells resulted in a mesenchymal to epidermoid transition, concurrent with increased N-cadherin and decreased \#946;-catenin levels. Comparison of the RNA and protein profiles of SCC9 cells and plakoglobin-expressing SCC9 transfectants identified a number of differentially expressed proteins and transcripts, including the nonmetastatic protein 23 (Nm23). Here, we assessed the effect of plakoglobin on the expression and subcellular localization of Nm23. Following plakoglobin expression, Nm23-H1 mRNA and Nm23-H2 protein were increased. Confocal microscopy showed membrane colocalization of Nm23 with plakoglobin, as well as with N-cadherin and \#945;-catenin. Consistent with these observations, co-immunoprecipitation experiments revealed that Nm23 interacts with plakoglobin, \#945;-catenin and N-cadherin and that these interactions occur most prominently in the cytoskeleton-associated pool of cellular proteins. We show that the plakoglobin-Nm23 interaction requires the N-terminal domain of plakoglobin, which is also involved in its interactions with \#945;-catenin. Our data suggest that the tumor suppressor activity of plakoglobin may be mediated, at least in part, by its interactions with, and regulation of the expression, stability and localization of Nm23.